The present invention relates to a distributed control system in which a plurality of control apparatuses are connected to a common transmission line, and especially to a filtering method and a filtering circuit used in the distributed control system, for selecting and receiving messages transmitted on the common transmission line.
In a process automation system or a factory automation system, many apparatuses each of which includes a processor, what is called “field apparatus”, are used for sensors to detect physical quantities such as pressure, temperature, or flow rate, motors to drive mechanical equipment, photo-sensors to detect positions. What is called a distributed control system controls an object system such as a plant by connecting the above field apparatuses, PCs (Programmable Controllers) for controlling the filed apparatuses, computers with input/output devices and display devices, and so on to a common transmission line so that those field apparatus, PCs, computers, etc., mutually send or receive messages via the transmission line.
In a distributed control system, for example, a physical quantity data from one sensor is simultaneously received by a plurality of motors, and each motor which has received the data independently executes processing by using the data from the sensor in real time. Therefore, data used for controlling of each field apparatus (referred to as a message) need to be transmitted in a very short time. Further, an extension or a change of a plant recently tends to be frequently carried out, which causes a frequent change in the composition of the control system for a plant. Accordingly, the easiness in extending or replacing field apparatuses or PCs has been strongly desired.
To satisfy the above requirements for a distributed control system, a conventional distributed control system uses a content-code transmission method in which each message with a content-code indicating the content of data to be transmitted is broadcasted on a common transmission line, and each node connected to the transmission line selects and receives the broadcasted message based on the content-code attached to the message. An example of the content-code using transmission method is disclosed in Japanese Patent Application Laid-Open Sho. 56-111353. Further, a distributed control system using the content-code transmission method is disclosed in Japanese Patent Application Laid-Open Hen. 1-124050. In this distributed control system, a received message includes an identifier of the kind of this message (message for on-line control or test) in addition to a code indicating the content of data to be transmitted and a node sending this message. Each node determines whether or not a received message is to be stored in the node, based on the above message-sending conditions included in the message.
According to recent development of a semiconductor technology, an intelligent function of a field apparatus has made further progress. Thus, although a conventional field apparatus simply detects or measures a physical quantity, a newer type of field apparatus performs complicated processing such as a self-diagnosis, a feed-back control, and so on. Consequently, it is required that results of a self-diagnosis performed in respective field apparatuses are collected by a computer which is a host apparatus of the field apparatuses, a plurality of parameters are set to each field apparatus, and similar net-work processing. Accordingly, in addition to the conventional data transmission in which messages of short data length such as data used to control an object system such as a plant, (referred to as control data), are transmitted on a common transmission line connecting a host computer and field apparatuses in a short transmission time, it has been required that messages with a long data length and of types different from those of the above control data, (referred to as information data), which need not be always transmitted in a short transmission time, are also transmitted along with control data on the transmission line in a recent distributed control system.
The above content-code transmission method is used to transmit control data to correspond to requirements of the easy extension and the synchronized data transmission of a distributed control system. On the other hand, since information data need not be simultaneously transmitted to a plurality of nodes, it is adequate to send information data by designating a message-receiving node, which is called a one-to-one transmission method. However, since the data length of information data is long, it takes much time for the node which has received the information data to process the information data, and the computing load of the node also increases.
In the content-code using transmission method in conventional techniques, data are broadcasted by attaching a content-code indicating the content of data to each data to be transmitted without designating a data-receiving node. Therefore, since data cannot be sent by designating the node receiving the data in the content-code transmission method, the content-code transmission method and the one-to-one transmission method cannot be used together.
Further, a distributed control system using the content-code transmission method has a problem in that since each control unit in the distributed control system receives all the message broadcasted on a transmission line, and determines whether or not each of the received messages is to be taken into the unit based on the attached content-code, the load for the transmission processing in each node is large. Especially, in a field apparatus performing component control and transmission processing with one CPU, if the load of the transmission processing to which the content-code transmission method is applied greatly increases, it affects processing of an application program to execute the component control. In the worst case, the timing or the period of the processing of an application program cannot be kept, which causes a failure in the normal control of a plant, etc. Furthermore, such a failure in the normal control frequently results in the serious deterioration in the safety of a plant or in the yield of a production line in a factory.